GeoScienceWorld Lithosphere Volume 2020, Article ID 8833404, 26 pages https://doi.org/10.2113/2020/8833404 Research Article The Anatomy and Origin of a Synconvergent Grenvillian-Age Metamorphic Core Complex, Chottanagpur Gneiss Complex, Eastern India 1 1 2 1 Nicole Sequeira , Souradeep Mahato, Jeffrey M. Rahl, Soumendu Sarkar, 1 and Abhijit Bhattacharya 1Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721 302, India 2Department of Geology, Washington and Lee University, 204 West Washington Street, Lexington, VA 24450, USA Correspondence should be addressed to Nicole Sequeira; [email protected] Received 14 June 2019; Accepted 26 January 2020; Published 0 June 2020 Academic Editor: Damian Nance Copyright © 2020 Nicole Sequeira et al. Exclusive Licensee GeoScienceWorld. Distributed under a Creative Commons Attribution License (CC BY 4.0). Amphibolite facies supracrustal rocks interleaved with granite mylonites constitute a shallowly dipping carapace overlying granulite facies anatectic basement gneisses in the Giridih-Dumka-Deoghar-Chakai area that spans ~11,000 km2 in the Chottanagpur Gneiss Complex (CGC). Steep N-trending tectonic fabrics in the gneisses include recumbent folds adjacent to the overlying carapace. The basement and carapace are dissected by steep-dipping sinistral shear zones with shallow/moderately plunging stretching lineations. The shear zones trend NNE in the north (north-down kinematics) and ESE in the south (south- down kinematics). Chemical ages in metamorphic monazites in the lithodemic units are overwhelmingly Grenvillian in age (1.0–0.9 Ga), with rafts of older domains in the basement gneisses (1.7–1.45 Ga), granitoids (1.4–1.3 Ga), and the supracrustal rock (1.2–1.1 Ga). P-T pseudosection analysis indicates the supracrustal rocks within the carapace experienced postthrusting – ° midcrustal heating (640 690 C); the Grenvillian-age P-T path is distinct from the existing Early Mesoproterozoic P-T path ° reconstructed for the basement gneisses. Quartz opening angle thermometry indicates that high temperature (~600 C) persisted during deformation in the southern shear zone. Kinematic vorticity values in carapace-hosted granitoid mylonites and in steep- dipping shear zones suggest transpressional deformation involved a considerable pure shear component. Crystallographic vorticity axis analysis also indicates heterogeneous deformation, with some samples recording a triclinic strain. The basement- carapace composite was extruded along an inclined channel bound by the steep left-lateral transpressional shear zones. Differential viscous extrusion during crustal shortening coupled with the collapse of the thickened crust caused midcrustal flow along flat-lying detachments in the carapace. 1. Introduction with extension-related processes such as slab rollback, intru- sion driven extension [13], or orogenic collapse under fixed The formation of Metamorphic Core Complexes (MCCs) is boundary conditions or slow plate convergence [14]. Searle conventionally related to extensional tectonic processes ([1] and Lamont [15] demonstrate that MCCs can form in and references therein), although proposed models vary in entirely compression regimes unaffected by any extensional the details [2–9]. In addition to regions dominated by exten- tectonism. sion, such as the Basin and Range province, MCCs have Synconvergent to postconvergent gravitational collapse been reported from accretionary margins [10] and in con- of a previously thickened orogenic crust [16–23] may lead vergent regimes [1, 11, 12]. Despite the convergent nature to the development of extensional shallow-dipping midcrus- of these tectonic settings, MCC development is associated tal detachment zones [19, 24–30] that may exhume deep- Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/doi/10.2113/2020/8833404/5293737/8833404.pdf by guest on 28 September 2021 2 Lithosphere seated lower crustal rocks within a MCC. Long et al. [31] U-Pb zircon geochronology [66–70] and pooled mona- demonstrate that regions of upper crustal thickening directly zite chemical ages [59] suggest the anatectic basement control the spatial location of synorogenic extension. Addi- gneisses of the CGC are the oldest (1.60–1.45 Ga) lithodemic tionally, vertical partitioning of strain in the crust during unit in the study area. This age range corresponds to high- convergence can localize extension in the upper crust and grade metamorphism-anatexis [61, 66, 69, 71] and emplace- contraction in the ductile lower crust (e.g., [1, 7, 32–37]). ment of felsic intrusives (Mukherjee et al. [67, 68]. Consider- The presence of regional scale strike-slip dominated able variations exist in the U-Pb zircon ages obtained by shear zones is noted in a number of MCCs [10, 38–45]. Mukherjee et al. [67–69] in the felsic lithologies in the Although these shear zones are known to aid exhumation Dumka-Deoghar area. Near-concordant and discordant U- of the deep crustal rocks [6, 46–54], limited data exist on Pb systematics in zircons from a charnockite gneiss/felsic the field relations of these shear zones and their role in the orthogneiss sample (AS-100) yield an upper intercept age of formation of MCCs [55]. 1450 Ma (emplacement age) and a lower intercept 943 Ma This work documents possibly the first MCC recognized age from the intermittent and weak metamorphic over- in peninsular India (cf. Figure 2 of [1]), based on detailed growth of zircon rims [67]. These authors suggest the weak structural mapping in the eastern part of the Chottanagpur Early Neoproterozoic overgrowths relate to a granulite- Gneiss Complex (CGC) (Figure 1). The proposed Early facies clockwise P-T path culminating with near-isothermal Neoproterozoic MCC lies in the foreland region of a contem- decompression in the retrograde sector. This contradicts poraneous convergent boundary between the Meso-/Neo- the earlier findings [60, 61, 66, 72] that the granulite facies proterozoic CGC and the Meso-/Neoarchean Singhbhum metamorphism in the basement gneisses is at least older than Craton in the south (Figure 1(a)) and is closely associated 1.4 Ga. It will be shown later that the field relationship and with steep-dipping transpressional shear zones. Kinematic mesoscale structures adopted by Mukherjee et al. [67] in their analyses coupled with crystallographic data, monazite dating, interpretation are contentious. and P-T pseudosection analyses illustrate the evolution of the In areas neighboring Dumka, Mukherjee et al. [68] report proposed MCC. two amphibole-biotite-gneiss/granite samples (AS-114A and AS-34/1) with a mean U-Pb zircon age of 1465 ± 17 Ma; this age is interpreted to correspond with the emplacement of the 2. Geological Background rocks. Based on the Lu-Hf isotopic compositions in zircon and major and trace element geochemistry of similar rocks, The Precambrian crystalline rocks of the CGC constitute a Mukherjee et al. [68] suggest that the protolith (ferroan A- vast area (~80,000 sq. km). The rocks may be classified into type granitoid) for the rocks was derived from a Paleoproter- three lithodemic units: (i) regionally extensive basement ozoic crustal source. Mukherjee et al. [69] determine U-Pb anatectic gneisses that include granulite facies felsic (char- zircon ages of two biotite-amphibole-garnet bearing felsic nockite-enderbite) orthogneisses, garnet-biotite-sillimanite- augen gneisses from Deoghar, e.g., 1709 ± 17 Ma for AS-37 K-feldspar-bearing metapelite, calc silicate granulite, mafic and 1626 ± 17 for AS-83A; the authors suggest that these granulite, massif anorthosites, and silica undersaturated ages represent the time of ultrahigh T granulite facies syenites; (ii) blastoporphyritic granitoids structurally varying metamorphism. from massive to foliated to mylonitic rocks predominant in There are no existing age data available in the amphibo- the CGC; and (iii) lower/middle-amphibolite facies supra- lite facies supracrustal rocks near Dumka. However, in the crustal rocks that include muscovite-biotite schist (with rare southern and central CGC, monazite chemical ages in the garnet and sillimanite), micaceous and ferruginous quartz- muscovite-biotite schists show two nonoverlapping age ites, amphibolites (hornblende-plagioclase±epidote), and groups: 1300–1200 Ma ages are found in strongly embayed minor amounts of metamarls and metadolomite ([56, 57] and fragmented cores mantled by younger (1050–880 Ma) and references therein). The high-grade gneisses and granu- metamorphic rims [59, 61, 62, 73, 74]. Metamorphic P-T lites occur as outcrop scale enclaves within the blastopor- path reconstructions in the supracrustal rocks in central phyritic granitoids that comprise the basement of the CGC CGC are not available. [58–61]. The supracrustal rocks are prominently exposed in The Early Neoproterozoic metamorphism is broadly E-trending belts that coincide with topographic highs along coeval with the emplacement (1.1–0.9 Ga) of the expansive and neighboring regional scale curvilinear ductile shear blastoporphyritic granitoids throughout the CGC ([66], and zones that trend E/ENE in central/northern CGC and references therein; [58, 61, 62, 71, 73–75]). Goswami and E/ESE in southern CGC [62] (Figure 1(a)). The three litho- Bhattacharyya [75] used petrographic and geochemical demic units are unconformably overlain by rift-related characters (major and trace elements, mineral chemistry, Gondwana basins that host Permo-Carboniferous to Early and 87Sr/86Sr ratios) to classify such granitoids in